Visitors consist of:
1) people who just read the site, but don't want to create an account to post.

2) "google-bots" (aka spiders or web crawlers) scanning the site for search indixing purposes. There are lots of these - at least as many as there are search engines on the web. These are harmless. This usually accounts for most of the visitors shown.

3) Other bots that are less desirable, like spam bots that scan the site for e-mail addresses to spam, and that attempt to create accounts for the purpose of spamming the forums. The graphic "verify the numbers" picture on account creation and login prevents these from getting in, as a bot can read the number and letters in the picture.

4) The least desirable - hack bots or hackers attempting to break into the site for vandalism purposes. Again the graphic verification helps, as does the other mods I have in place. We also run both the Sentinel and Protector security packages.

I forgot to point out, under the 1st group, that members who come to the site to read the forums, but don't log in until they try to post or do something requiring they be logged in, would also show up as visitors until they log in.